A synchronous machine operable in both starting modes and generating modes is disclosed in Glennon et al U.S. Pat. No. 5,068,590. The synchronous machine has a permanent magnet generator, an exciter and a main generator. In the generating mode, constant frequency AC power is generated by the synchronous machine from motive power provided by a prime mover, such as a jet engine. In the starting mode, the synchronous machine is accelerated by providing variable frequency electrical power to the main generator.
When a synchronous machine such as the one described above is operated in the starting mode, the frequency of the power provided to the armature winding of the main generator needs to be controlled in accordance with the relative position between the armature winding of the main generator and the field winding. Typically, some type of position sensor is used to detect the position of the synchronous machine rotor so that the variable frequency power is provided to the main generator armature winding at the appropriate intervals. For example, in Lafuze U.S. Pat. No. 3,902,073, three Hall effect sensors are used to determine the rotor position. Other types of sensors, such as resolvers or optical encoders, could be used to determine rotor position.
Rotor position may be determined without use of a sensor by detecting the back EMF generated in an unenergized phase winding of an armature winding of a synchronous machine. Since the magnitude of the back EMF is proportional to the rotational speed of the synchronous machine, being relatively low at low speed, it may be difficult to reliably start the synchronous machine when rotor position detection is based on back EMF. Detecting rotor position based on back EMF may be further complicated when the synchronous machine is started utilizing pulse-width modulated drive signals for current-limiting purposes.